Plasma has long been employed to process substrates (e.g., wafers or glass panels) to produce electronic products (e.g., integrated circuits or flat panel displays). In the processing of a substrate, plasma may be employed to etch or deposit material. Generally speaking, plasma processing involves disposing a substrate on a suitable work piece holder, such as a chuck. An RF (radio frequency) energy source may be employed to ignite a process source gas inside a plasma processing chamber, forming a plasma for processing the substrate. In the case of an inductively coupled plasma processing chamber, this RF plasma-generation energy source is typically implemented by an RF power supply supplying RF energy to an inductive coil. In the following discussion, wafers and inductively coupled plasma processing chambers are employed as examples. It should be understood, however, that the invention is not limited to these particular examples.
When the RF energy source is on, plasma can be generated and sustained, which may involve the generation of the charged species and radical neutral species. During processing, charged species from the plasma tend to impart an electrical charge on features on the substrate surface. This electrical charge on the wafer may, in some cases, undesirably result in the alteration of the desired topography of the features and may even lead to device damage. For example, charged species may be attracted to or repelled from the charged sidewalls of features on the wafer surface, resulting in undercut features after the etch is completed. The electrical charge on the wafer may also cause electrical stress among features and layers of the wafer. Furthermore, energetic charged and neutral species striking the charged wafer may cause structural damage (such as voids or dislocations) to features formed in the upper layer of the wafer.
When the RF energy to the inductive coil is turned off, the plasma enters an afterglow period during which, depending on the chemistry employed, may cause the wafer to continue to be processed (e.g., etched and/or deposited), albeit using a modified mechanism. The etching and/or deposition activity that occurs during the afterglow period has been successfully utilized in many processes to attenuate the potentially damaging effects of charged species striking the wafer, for example.
Once the species (both charged species and reactive neutral species) are expended or evacuated, only unenergized processed gases remain. In the absence of plasma enhanced species, processing tends to cease or proceed only minimally. Accordingly, useful processing in the afterglow state tends to be limited in duration.
Because of the beneficial effect of afterglow processing, pulsing has been employed to alternately turn the plasma on and off. Generally speaking, pulsing may be performed with either the source RF power supply (i.e., the RF power supply employed mainly for plasma ignition and sustaining) or the bias RF power supply that is employed to bias the chuck on which the wafer is disposed.
To clarify terminologies, in an inductive chamber, the RF energy source that supplies RF energy to the inductive coil tends to be the main plasma ignition and sustaining power supply. This power supply is referred to herein as the source RF power supply for inductively coupled chambers. On the other hand, the RF energy source that provides RF power to the chuck to primarily control the sheath voltage and ion energy is referred to herein as the bias RF power supply.
In a capacitively coupled chamber that employs multiple RF frequencies, the RF energy source that supplies the high frequency RF signal tends to be the main plasma ignition and sustaining power supply. This power supply is referred to herein as the source RF power supply for capacitively coupled chambers. On the other hand, the RF energy source that provides the lower frequency RF signal to the chuck to control the bias energy is referred to herein as the bias RF power supply.
If the plasma is pulsed, only the source RF power supply or only the bias RF power supply or both may be pulsed. If both RF power supplies are pulsed, the pulsing of both the source RF power supply and the bias RF power supply may be performed asynchronously or synchronously (and if synchronously, may be in phase or out of phase). In the prior art, the source RF power supply and the bias RF power supply tend to be pulsed both synchronously and in-phase. As the term is employed herein, two RF signals are said to be synchronous if there is a pulse of the first signal for every pulse of the second signal and vice versa. On the other hand, two RF signals are said to be in phase if their pulses have the same rising edge and lowering edge.
FIG. 1 shows two synchronous and in-phase RF signals 102 and 104. RF signal 102 represents the source RF signal supplied by the source RF power supply for plasma ignition and sustaining while RF signal 104 represents the bias RF signal supplied by the bias RF power supply for controlling the sheath that exists between the plasma and the wafer during processing in order to control the energy of particles impacting the wafer. When source RF signal 102 is turned on (shown by reference 106), there exists a time delay, Td, before the plasma ignites. Plasma ignition is denoted in FIG. 1 by reference number 108.
If the bias RF signal is on during the duration Td when no or little plasma exists in the chamber, the chuck may be unduly biased by the on state of the bias RF signal pulse. The high bias condition, which is characterized by a high bias voltage existing on the chuck in the absence of a high density plasma in the chamber, may cause particles and/or species to impact the chuck and/or the wafer at high velocity, leading to bombardment damage. The duration of this high bias condition is shown in FIG. 1 by the reference THB1.
FIG. 2 illustrates the situation wherein the bias RF signal pulse is time shifted relative to the source RF signal pulse in an attempt to avoid the aforementioned high bias condition. In the example of FIG. 2, after the source RF signal 202 is turned on at edge 206, the plasma ignites after a delay period Td. As before, source RF signal 202 is turned off after some time (as signified by reference number 210). After source RF signal is turned off at edge 210, the plasma in the chamber is extinguished except for the residual afterglow species which decay away as shown by reference number 212 as the chamber continues to be evacuated by exhaust pumping.
In FIG. 2, the bias RF signal pulse is delayed by a time period Td to ensure that the bias RF signal pulse is turned on only after high density plasma is ignited in order to avoid the aforementioned high bias condition. However, since the bias RF signal pulse is kept at the same duration (e.g., the bias RF signal pulse is phase-shifted to account for the ignition delay but its pulse has the same duration as the source RF signal pulse), the same high bias condition may exist after the source RF power supply is turned off. This is because the presence of bias RF signal pulse on the chuck when high density plasma is no longer actively generated in the chamber may result in the aforementioned high bias condition (shown in FIG. 2 by the reference THB2). This high bias condition, as mentioned, may potentially damage the wafer and/or the chuck due to excessive bombardment.